Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance, where more than one radio, supported by digital signal processing are used. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over or collecting the received signal from the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF circuitry that implements beamforming and usually includes a combiner and may further include switches, controllable phase shifters, and in some cases controllable amplifiers and/or controllable attenuators.
The term “layer” as used herein, or specifically, single layer is defined as a transmitted signal carrying a data stream from a single transmitting antenna. Multi-layer transmission is defined as co-frequency simultaneous multiple data streams radiated over multiple antennae using pre-coding that facilitates spatial separation, in a way that allows the various co-frequency receivers each to receive their own data stream while suppressing the others.
The term “Receiving Radio Distribution Network” or “Rx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above.
The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employs two or more antennas per channel (N is the number of channels and M is the total number of antennas and M>N). This architecture employs a beamformer for each channel so that two or more antennas are combined for each radio circuit that is connected to each one of the channels.
The term “average” as used herein means summation of a series of L values without necessarily dividing by the L, the result being subjected to scaling or normalization.
In hybrid MIMO RDN receiving systems, when the phases of the received signals from each antenna are properly adjusted with respect to one another, the individual signals may be passively combined and result in an improved Signal to Interference plus Noise Ratio (SINR) for the receiving system. A drawback of passive combining schemes is that if the input signals or noises have different powers, theses imbalances may significantly affect the SINR of the combined signal at the output of the combiner of the beamformer and may therefore degrade the gain of the beamformer.